Two NASA astronauts have begun the first of at least two spacewalks aimed at replacing a malfunctioning pump module that is part of the cooling system for the International Space Station. The astronauts, Col. Michael S. Hopkins of the Air Force and Richard A. Mastracchio, stepped out of an air lock on Saturday for what is scheduled to be six and a half hours of work before they return to the station. NASA reported the astronauts were about 25 minutes ahead of their timeline.